Resins for toner of electrophotography and method for manufacturing the same

ABSTRACT

A resin for toner of electrophotography comprises a urethane-modified polyester resin (D) having a glass transition temperature ranging from 40° to 75° C., which is obtained by reacting a resin mixture composed of a polyester resin (A) having a number-averaging molecular weight of 1,000 to 15,000 and a hydroxyl value of 10 to 100 and a polyester resin (B) having a number-average molecular weight ranging from 1,000 to 5,000 and a sum of acid value and hydroxyl value of less than 10, in a weight ratio of (A)/(B) ranging from 20:80 to 60:40, with 0.3 to 0.99 molar equivalent of an isocyanate compound (C) per one molar equivalent of hydroxyl group of the polyester resin (A). 
     The resin for toner is excellent in grindability and storage stability and it provides toners for electrophotography having good anti-blocking properties and good fixing properties whereby good images free of fogging can be obtained.

This is a division of application Ser. No. 359,217, filed on May 31,1989, now U.S. Pat. No. 4,981,923.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a resin for toner ofelectrophotography, a method for manufacturing the same and a tonercomposition containing such a resin, and more specifically to a resinfor toner which shows high humidity resistance and is stable tohumidity.

2. Prior Art

Electrophotography is a technique for obtaining images by forming anelectric latent image on a light-sensitive material according to variousmeans utilizing a photoconductive substance, developing such an imagewith a toner, optionally transferring the toner image to a support forimage such as paper and then fixing the image by means of, for instance,a heating roll to thus obtain an electrophotograph. Various kinds ofresins have been developed to use them in such toners to be employed inelectrophotography. Among these, polyester resins have presently beenused widely as resins for toners which are fixed with a heating rollbecuase they show good wettability to transfer paper and can disperseuniformly, therein, additives for toner such as carbon black when theyare melted by applying heat and further, they can impart excellentfixing properties to the resulting toner (e.g. U.S. patent applicationNo. 110,695 based on PCT JP87/00064, Jap. Pat. Kokai Koho Nos.60-263950, 61-86760 and 62-70860).

In the electrophotography, it is always required to provide clear andstable images from the viewpoint of sending information.

However, in the conventional toners, the polyester resins used thereinhave, at their ends, hydrophilic groups such as carboxyl groups andhydroxyl groups derived from monomers constituting the polymers and,therefore, flow properties and shelf stability as well as chargingproperties of the toners are greatly influenced by change inenvironmental conditions, in particular, change in humidity. This makesit impossible to always obtain clear images according toelectrophotography technique. As means for solving this problem, therehas been proposed, for instance, a method for eliminating influences bysuch change in properties of the toners which comprises adding, to sucha toner composition, additives such as fine powders of hydrophobicsilica. This method can improve the properties of the toner in somedegree, but is accompanied by a new problem that the surface of alight-sensitive material for copying apparatuses is impaired. Therefore,the foregoing problem has not yet been solved thoroughly.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a resin for toner usedin electrophotography which shows good grindability in the preparationof toner, can stand against environmental cahgnes, particularly humiditychange and is hence excellent in anti-blocking properties.

Another object of the present invention is to provide a tonercomposition for electrophotography which has good fixing properties andexcellent humidity resistance and which can provide good images free offogging.

According to the present invention, there is provided a resin for tonerof electrophotography which comprises a urethane-modified polyesterresin (D) having a glass transition temperature ranging from 40° to 75°C., which is obtained by reacting a resin mixture composed of apolyester resin (A) having a number-average molecular weight of 1,000 to15,000 and a hydroxyl value of 10 to 100 and a polyester resin (B)having a number-average molecular weight ranging from 1,000 to 5,000 anda sum of acid value and hydroxyl value of less than 10, in a weightratio of (A)/(B), ranging from 20:80 to 60:40, with 0.3 to 0.99 molarequivalents of an isocyanate compound (C) per one molar equivalent ofhydroxyl group of the polyester resin (A).

According to another aspect of the present invention, a tonercomposition for electrophotography comprising the afore-mentionedurethane-modified polyester resin (D) and various additives commonlyused is provided.

According to a further aspect of the present invention, there isprovided a method for preparing the foregoing urethane-modifiedpolyester resin (D) which comprises reacting the foregoing resin mixturecomposed of the polyester resin (A) and the polyester resin (B) with theisocyanate compound (C) while kneading the reaction mixture using ameans for kneading equipped with a built-in screw.

DETAILED DESCRIPTION OF THE INVENTION

The polyester resin (A) herein means those obtained by polycondensationof a polycarboxylic acid and a polyhydric alcohol. Examples of suchpolycarboxylic acids include such aliphatic dibasic acids an malonicacid, succinic acid, glutaric acid, adipic acid, azelaic acid, sebacicacid and hexahydrophthalic anhydride; such aromatic dibasis acids asphthalic anhydride, phthalic acid, terephthalic acid and isophthalicacid; and lower alkyl esters thereof.

Moreover, examples of such polyhydric alcohols are such diols asethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol,1,3-butylene glycol, 1,4-butylene glycol, 1,6-hexanediol, neopentylglycol, diethylene glycol, dipropylene glycol, hydrogenated bisphenol Aand bisphenol A-propylene oxide adducts; and such triols as glycerin,trimethylol propane and trimethylol ethane. Preferred are bisphenolA-propylene oxide adducts among others.

As the method for polycondensation, there may be used, for instance,commonly known high temperature polycondensation and solutionpolycondensation.

The molecular weight of the polyester resin (A) ranges from 1,000 to15,000 expressed in number-average molecular weight. This is because ifit is less than 1,000, heat resistance of the resultanturethane-modified polyester resin (D) is insufficient, the molecularweight of the urethane-modified resin (D) is lowered when it is meltedand kneaded during preparation of toners whereby fog is caused duringelectrophotography and resistance to offset of the resultant toner isimpaired, while if it is more than 15,000, fixing properties of theresulting toner are lowered.

In the invention, the hydroxyl value of the polyester (A) is limited to10 to 100. This is because if it is less than 10, resistance to offsetof the resultant toner is impaired while if it is more than 100, heatresistance of the resulting urethane-modified resin is lowered. The acidvalue of the polyester (A) is, preferably, less than 2 and morepreferably, less than 1.

The polyester resins (B) as used herein are those obtained according inthe same manner as described above associated with the polyester resin(A), which have a number-average molecular weight ranging from 1,000 to5,000 and the sum of acid value and hydroxyl value of less than 10,preferably less than 8. The acid value and hydroxyl value are,preferably, in the range of 0 to 4, respectively. The same examples asthose listed above associated with the polyester resin (A) may also beused in this case as polycarboxylic acids and polyhydric alcohols.Particularly preferred polyester resins (B) are condensates of bisphenolA-propylene oxide adducts; and those of trimethylol propane and dimethylterephthalate.

The molecular weight of the polyester resin (B) ranges from 1,000 to5,000 and preferably, 2,000 to 4,000 expressed in number-averagemolecular weight. If it is less than 1,000, resistance to offset of thetoner obtained by using the resulting urethane-modified resin (D) islowered while if it is more than 5,000, fixing properties of the tonerand grindability thereof during manufacturing the same is impaired.

Moreover, if the sum of the acid value and the hydroxyl value of thepolyester resin (B) is more than 10, flowability and shelf stability aswell as charging properties of the resulting toner greatly varydepending upon environmental changes, in particular change in humiditywhereby fog is caused during electrophotography and hence it isimpossible to always obtain clear images, since the carboxyl groups andhydroxyl groups at the termini of the resin (B) are stronglyhydrophilic. On the other hand, if the hydroxyl value increase to morethan 10, an ester interchange reaction is easy to occur and, as aresult, molecular weight distribution becomes narrow and fixingproperties and resistance to offset of toners are impaired.

The weight ratio of the polyester resin (A) to the polyester resin (B)ranges from 20:80 to 60:40. This is because if it is less than 20:80, inother words the amount of the polyester resin (A) is too small,resistance to offset of the resulting toner is lowered and fixingtemperature range becomes narrow, while if the weight ratio is more than60:40, the grindability is impaired.

In addition, as the isocyanate compounds (C) used in the invention theremay be mentioned, for instance, hexamethylene diisocyanate, isophoronediisocyanate, tolylene diisocyanate, diphenylmethane diisocyanate,xylylene diisocyanate and tetramethylxylylene diisocyanate.

The isocyanate compound (C) is usually used in an amount ranging from0.3 to 0.99 molar equivalent, preferably 0.5 to 0.95 molar equivalentper one molar equivalent of hydroxyl group of the polyester resin (A).If the isocyanate compound is used in an amount less than 0.3 molarequivalent, the resistance to offset of the resulting toner is lowered,while if it exceeds 0.99 molar equivalent, the viscosity of reactionsolution is greatly increased during the reaction and, in the worstcase, the viscosity of the urethane-modified polyester resin (D) becomestoo high to stir the reaction solution, in other words there is causedso-called ultra-high viscosity phenomenon which is not practicallyfavorable.

The urethane-modified polyester resin (D) may be obtained, for instance,according to the following method. In other words, it may be obtained byinstroducing as isocyanate compound into a mixture of the polyesterresins (A) and (B) in one lot, by portionwise addition or continuousaddition at a temperature of 60° to 190° C. and reacting these at thattemperature for several minutes to several hours.

In a preferred embodiment, the isocyanate compound (C) is reacted withthe mixture of the polyester resin (A) and polyester resin (B) whilecontinuously kneading these ingredients in a single screw or twin-screwextruder. In particular, the use of the twin-screw extruder makes itpossible to enhance the mixing and dispersion effects of the resincomponents and the isocyanate compound and to extend the averageresidence time of the reaction products in a reaction zone of theextruder. As such extruder particularly preferred is a twin-screwengaging type extruder in which these two screws rotate in the samedirection.

The precision of a feeder for supplying a mixture of the polyesterresins (A) and (B) in a desired ratio to an extruder and that of adispensing pump are desirably as high as possible. The amount of theresin supplied to the extruder is determined in response to the averageresidence time thereof in the extruder.

As specifications of the extruder used, it is required to extend theaverage residence time of the contents of the extruder in order tosufficiently react the polyester resin (A) in the mixture with theisocyanate compound (C). To achieve the purpose, the raito L/D of theextruder (the ratio of the length (L) of the screw to the diameter (D)thereof) is desirably limited to not less than 10 and preferably notless than 20 and not more than 60. More specifically, the screw of theextruder is divided into three zones from an inlet for supplying thepolyester resins (A) and (B). i.e., a feed zone, a volatile componentsremoving zone and a reaction zone, and it is preferred to design thescrew so that the length of each zone is 3 to 20 for the feed zone; 0 to10 for the volatile components removing zone; and 20 to 40 for thereaction zone as expressed in L/D. It is also preferred to design thescrew so that the temperature of each zone can independently beestablished during operating the extruder. In general, the temperatureof these zones are desirably controlled precisely to 30° to 100° C. forthe feed zone; 100° to 250° C. for the volatile components removingzone; and 120° to 220° C. for the reaction zone.

The volatile components removing zone may be omitted if there are a verylittle or no volatile componenets and/or solvents in the startingmaterial and correspondingly the lengths of the feed zone and thereaction zone may be elongated.

A cylinder of the extruder may be heated according to heating with anelectric heater or heating with a heat transfer medium. In general, theheating with a heat transfer medium which permits uniform heating ispreferred compared with the heating with an electric heater which isliable to cause local heating, but in the present invention the bothheating means can be employed.

The rotation number of the screw has a close correlation with the feedrate of the starting resins, but it is preferred to control it so thatthe average residence time of the contents in the reaction zone is inthe range of from 15 to 25 minutes.

The glass transition point of the urethane-modified polyester resin (D)is limited in the present inveniton to 40° to 75° C. and preferably 50°to 60° C. This is because if the glass transition point thereof is lessthan 40° C., the antiblcok properties of the resulting toner isimpaired, while if it exceeds 75° C., the fixing properties of the toneris lowered.

The toner composition for electrophotography comprises the foregoingresin (D) and proper coloring agents and charge controlling agents aswell as fillers are incorporated thereinto according to need.

Examples of suitable coloring agents are carbon black, Aniline Blue,Alkoyl Blue, chrome yellow, Ultramarine Blue, Quinoline Yellow,Methylene Blue, Phthalocyanine Blue, Malachite Green, Rose bengale andMagnetite.

As the charge controlling agents, any conventionally known ones may beincorporated into the toner composition of the invention. Examplesthereof include Nigrosine, triphenylmethane type dyes and chromiumcomplex of 3,5-di-tert-butyl salicylic acid.

As the fillers, there may be used any conventionally known ones, forinstance, colloidal silica, zinc stearate, low molecular weightpolypropylene, stearic acid amide and methylenebisstearoamide.

The foregoing resin and other additives are subjected to premixing witha Henschel mixer or the like, then are melted and kneaded at 100° to180° C. with a kneader, followed by pulverizing the resulting mass andclassifying to obtain particles having a size of 5 to 15 microns whichmay be used as a toner for electrophotography.

The present invention will hereunder be explained in more detail withreference to the following Examples and Preparation Examples. In thefollowing Examples and Preparation Examples, the term "part(s)" denotes"part by weight" unless otherwise specified.

PREPARATION EXAMPLES A-I TO A-V

In these Preparation Examples, polyester resins (A) are to be prepared.

To a 5 liter four-necked flask equipped with a reflux condenser, a waterseparator, a conduit for introducing nitrogen gas, a thermometer and astirrer there were charged a polybasic acis and a polyhydric alcohol aslisted in Table 1 and they were subjected to dehydrationpolycondensation at 220° to 240° C. while introducing nitrogen gas intothe flask. The reaction was stopped at an acid value of not more than 1to obtain polyester resins A-I to A-V.

PREPARATION EXAMPLES B-I TO B-VII

In these Preparation Examples, polyester resins (B) are prepared.

To a 5 liter four-necked flask equipped with a reflux condenser, a wateror alcohol separator, a conduit for introducing nitrogen gas, athermometer and a stirrer there were charged a polybasic acids or alower alkyl ester thereof and a polyhydric alcohol as listed in Table 2and they were subjected to dehydration or dealcohol polycondensation at220° to 240° C. while introducing nitrogen gas into the flask. Thereaction was stopped at a time where a desired acid value or hydroxylvalue was attained to obtain polyester resins B-I to B-VII.

PREPARATION EXAMPLES 1 TO 15

In these Preparation Examples, urethane-modified polyester resins (D)were prepared.

The polyester resins (A) and (B) obtained above were separatelypulverized to control the particle size of the resins to 0.5 to 1 mm andwere dispensed so as to meet the mixing ratio shown in Table 3 and theseresin particles were premixed in a Henschel mixer. The premixed resinswere modified with urethane in a twin-screw extruder (available from THEJAPAN STEEL WORKS, LTD. under the trade name of TEX-30; L/D=30). Asextrusion conditions, the temperature of a cylinder of the extruder wasset up so that the temperature of the resins was established to 170° C.

The length (L/D) and temperature of each of the feed zone, volatilecomponents removing zone and reaction zone are set up as follows:

    ______________________________________                                                    L/D (30)  Temperature, °C.                                 ______________________________________                                        Feed zone     6           160                                                 Volatile components                                                                         2           170                                                 removing zone                                                                 Reaction zone 22          170                                                 ______________________________________                                    

The modification of the resins with urethane was carried out bycontinuously supplying the premixed resins to an extruder in the rate of4 kg/hr with a constant rate feeder to melt and knead the resins,reducing the pressure in the extruder to remove volatile componentspossibly present in the resins through a venting hole formed on theextruder and then continuously supplying a desired amount of aisocyanate compound (C) (tolylene diisocyanate) as listed in Table 3 tothe extruder through an injection nozzle with a constant rate feeder tothus react the resin with the isocyanate compound (C) while controllingthe average residence time of the reaction product in an area of theextruder behind the injection nozzle to 20 minutes. The resultingurethane-modified polyester resin (D) was cooled and granulated.

EXAMPLE 1 TO 15

After dispersing and mixing, in a Henschel mixer, 6 parts by weight ofcarbon black MA-100 (available from MITSUBISHI CHEMICAL INDUSTRIES,LTD.), 2 parts by weight of a charge controlling agent (available fromHODOGAYA CHEMICAL CO., LTD. under the trade name of Spiron Black TRH), 2parts by weight of polypropylene type wax (available from SANYO CHEMICALINDUSTRIES, LTD. under the trade name of Biscol 550P) and 3 parts byweight of bisamide type wax (available from LYON ARMOUR CO., LTD. underthe trade name of Armowax EBS) per 100 parts by weight of theurethane-modified polyester resins obtained in Preparation Examples 1 to15, the mixtures were melted and kneaded at 160° C. in a twin-screwkneader PCM 30 (available from IKEGAKI TEKKO LTD.) to obtain massivetoner compositions.

These compositions were granulated with a hammer mill, followed byfinely pulverizing them with a jet pulverizer (IDS 2 Type, availablefrom NIPPON NEWMATIC CO., LTD.) and then classifying by airboneclassification to obtain toner particles having an average particle sizeof 10 microns (3% by weight ≦5 microns; 2% by weight ≧20 microns).Properties of the resulting toner particles are summarized in thefollowing Table IV.

4 Parts by weight of the toner was mixed with 100 parts by weight offerrite carrier F-150 (NIPPON IRON POWDER CO., LTD.) to obtain atwo-component developer.

Using a commercially available magnetic brush type copying apparatus(available from TOSHIBA ELECTRIC EQUIPMENT CORPORATION under the tradename of REODRY 8411), a copying test was performed at room temperature(25° C.), in an environment of 50% and 80% RH (relative humidity) whilechanging the heat roller temperature. The results of evaluation offixing temperature range and resistance to migration of vinyl chlorideplasticizers are summarized in Table IV.

Also shown in Table IV are the evaluation results on density change ofimages, the extent of fog generated and the like observed after 50,000copies were obtained.

In the following Tables, each note is as follows:

1) Bisphenol A-(2,2)-propylene oxide adduct (available from MITSUITOATSU CHEMICALS, INC.).

2) Determined according to JIS K5400.

3) This means number-average molecular weight determined by gelpermeation chromatography (GPC) in which tetrahydrofuran is used as aneluent and a refractometer is used as a detector using monodispersestandard polystyrene as the standard.

4) Weight-average molecular weight determined according to the samemanner as in Item 3).

5) Determined according to pyridine-acetic anhydride technique.

6) Glass transition point determined by Differential ScanningCalorimeter (DSC).

7) Tolylene diisocyanate.

8) The toner prepared was allowed to stand in an environment of 50° C.and 50% RH or 80% RH for 24 hours and the extent of aggregation wasvisually observed and evaluated according to the following four-stageevaluation.

⊚: Aggregation was not observed at all.

: Particles were slightly aggregated, but the aggregates were peptizedby lightly shaking a container.

Δ: Some of the aggregates were not peptized even if the container wassufficiently shaken.

×: The particles were completely conglomerated.

9) The grindability was evaluated from the yield of particles having aparticle size ranging from 5 to 20 microns obtained by finelypulverizing and classifying them, according to the following four-stageestimation.

A: Yield of not less than 90%; excellent.

B: Yield of not less than 80% and less than 90%; good.

C: Yield of not less than 70% and less than 80%; fair.

D: Yield of less than 70%; failure.

10) This means a minimum surface temperature of a heat fixing roll atwhich the so-called offset phenomenon is caused, in other words, moltentoners are adhered to the heat fixing roll and again fixed on copyingpaper at that temperature.

11) This means a minimum surface temperature of the heat fixing rollrequired for maintaining a toner layer in a residual weight rate of morethan 80% even after rubbing, 50 times with a sand eraser, the tonerlayer on the image of solid black portions having a size of 2 cm×2 cm,under a load of 125 g/cm² utilizing a GAKUSHINE Type tester for fastnessto crocking (available from DAIEI KAGAKU SEIKI MANUFACTURING CO., LTD.).

12) On a solid black portion of 5 cm×5 cm, there was superposed acommercially available polyvinyl chloride sheet (content of dioctylphthalate=50% by weight; available from MITSUI TOATSU CHEMICALS, INC.)the assembly was allowed to stand at 50° C. for 24 hours under a load of20 g/cm² and thereafter the polyvinyl chloride sheet was peeled off atroom temperature. The migration of the toner to the polyvinyl chloridesheet was observed with the naked eyes at this stage and the extentthereof was evaluated according to the following four-stage evaluation:

⊚: The migration of toner and dyes was not observed at all.

: Only dyes were migrated to the polyvinyl chloride sheet.

Δ: A part of the toner was migrated.

×: Most of the toner was migrated.

13) The degree of blackness of the solid black portions on imagesobtained after forming 50,000 copies was estimated with the naked eyes.

14) The extent of contamination of the background white portion(non-image portion), due to the adhesion of toners, observed on the copyobtained after forming 50,000 copies was inspected with the naked eyesand was estimated according to the following fourstage evaluation:

⊚: No background contamination.

: Observed slight background contamination.

Δ: Observed substantial background contamination.

×: Observed severe background contamination.

As seen from the results listed in Table 4, toner compositions obtainedaccording to the present invention are excellent in their grindabilityduring their preparation, further during storage, can stand against anyenvironmental changes, in particular, change in humidity and, therefore,are excellent in anti-blocking properties and hence good fixingproperties. Thus, according to the toner composition of this invention,good images free of fogging can be obtained.

                  TABLE 1                                                         ______________________________________                                        Polyester Resin (A) No.                                                                      A-I    A-II   A-III A-IV  A-V                                  ______________________________________                                        KB300K.sup.(1) (wt. part)                                                                    79     91     107   125   140                                  Neopentyl glycol                                                                             14     13     12    11    9                                    (wt. part)                                                                    Trimethylol propane                                                                          3      5      7     10    12                                   (wt. part)                                                                    Isophthalic acid (wt. part)                                                                  54     67     78    83    91                                   OH/COOH        1.22   1.20   1.18  1.17  1.15                                 Hydroxyl value.sup.(2)                                                                       130    85     20    11    6                                    (KOHmg/g)                                                                     Mn.sup.(3)     800    1980   7700  14200 18000                                Mw.sup.(4)     2400   6100   23800 44000 61000                                ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Polyester                                                                     Resin (B) No.                                                                           B-I    B-II   B-III                                                                              B-IV  B-V  B-VI B-VII                            ______________________________________                                        KB300K    1000   1200   610  790   540  740  810                              (wt. part)                                                                    Trimethylol                                                                             --     4.0    3.1  4.2   3.6  5.9  6.6                              propane                                                                       (wt. part)                                                                    Dimethyltele-                                                                           870    960    423  520   --   --   550                              phthalate                                                                     (wt. part)                                                                    Isophthalic acid                                                                        --     --     --   --    300  440  --                               (wt. part)                                                                    Mn        820    1250   3000 4930  3100 3400 6100                             Mw        1800   2850   6600 10850 6880 7900 14000                            Acid value.sup.(5)                                                                      <1     <1     <1   <1    36   11   <1                               (KOHmg/g)                                                                     Acid value +                                                                            9      8      7    5     44   35   4                                Hydroxyl value                                                                (KOHmg/g)                                                                     Tg.sup.(6) (°C.)                                                                 40     45     53   56    62   60   58                               ______________________________________                                    

                                      TABLE 3                                     __________________________________________________________________________                       Preparation Example No.                                                       1   2   3   4   5   6   7   8                              __________________________________________________________________________    Polyester resin (A)                                                           No.                A-I A-II                                                                              A-II                                                                              A-III                                                                             A-IV                                                                              A-II                                                                              A-II                                                                              A-II                           Wt. part           40  40  40  20  60  40  40  70                             Polyester resin (B)                                                           No.                B-III                                                                             B-I B-III                                                                             B-III                                                                             B-III                                                                             B-II                                                                              B-IV                                                                              B-III                          wt. part           60  60  60  80  40  60  60  30                             Urethane-modified polyester resin (D)                                         TDI.sup.(7)        12.9                                                                              8.5 8.5 1.2 1.5 8.5 8.5 14.9                           NCO/OH             0.8 0.8 0.8 0.9 0.7 0.8 0.8 0.8                            Kneading temperature (°C.)                                                                170 170 170 170 170 170 170 170                            Residence time (min.)                                                                            20  20  20  20  20  20  20  20                             Mn                 3100                                                                              900 3200                                                                              3100                                                                              3400                                                                              1300                                                                              5000                                                                              Ultra-high                     Mw                 45000                                                                             46000                                                                             125000                                                                            108000                                                                            135000                                                                            98000                                                                             115000                                                                            viscosity                      Tg (°C.)    47  47  62  61  64  49  57  73                             __________________________________________________________________________                       Preparation Example No.                                                       9    10   11    12  13  14  15                             __________________________________________________________________________    Polyester resin (A)                                                           No.                A-II A-II A-II  A-II                                                                              A-II                                                                              A-II                                                                              A-V                            Wt. part           10   40   40    40  40  40  40                             Polyester resin (B)                                                           No.                B-III                                                                              B-III                                                                              B-III B-V B-VI                                                                              B-VII                                                                             B-III                          wt. part           90   60   60    60  60  60  60                             Urethane-modified polyester resin (D)                                         TDI.sup.(7)        2.1  2.1  10.6  8.5 8.5 8.5 8.5                            NCO/OH             0.8  0.2  1.0   0.8 0.8 0.8 0.3                            Kneading temperature (°C.)                                                                170  170  170   170 170 170 170                            Residence time (min.)                                                                            20   20   20    20  20  20  20                             Mn                 3000 3000 Ultra-high                                                                          3800                                                                              5000                                                                              6200                                                                              Ultra-high                     Mw                 39000                                                                              26000                                                                              viscosity                                                                           110000                                                                            75000                                                                             156000                                                                            viscosity                      Tg (°C.)    54   52   75    72  67  65  73                             __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________                  Example No.                                                                   1     2    3    4    5    6    7    8                           __________________________________________________________________________    Properties of toner particles                                                 Anti-blocking.sup.(8) (RH 50%)                                                              ×                                                                             ×                                                                            ⊚                                                                   ⊚                                                                   ⊚                                                                   ⊚                                                                   ⊚                                                                   ⊚            Anti-blocking (RH 80%)                                                                      ×                                                                             ×                                                                            ⊚                                                                   ⊚                                                                   ⊚                                                                   ⊚                                                                   ⊚                                                                   ⊚            Grindability.sup.(9)                                                                        A     A    A    A    A    A    A    D                           Fixing property                                                               Offset occurence                                                                            190   190  240< 240< 240< 240< 240< 240<                        temperature.sup.(10) (°C.)                                             Lower temp. of fixing (°C.)                                                          140   130  140  140  140  120  150  200                         Temp. range of fixing (°C.)                                                           50    60  100< 100< 100< 120< 90<  40<                         Resistance to dioctyl                                                                       Δ                                                                             ×                                                                            ⊚                                                                   ⊚                                                                   ⊚                                                                   ⊚                                                                   ⊚                                                                   ⊚            phthalate.sup.(12)                                                            Image                                                                         Image intensity.sup.(13)                                                                    Intense                                                                             Intense                                                                            Intense                                                                            Intense                                                                            Intense                                                                            Intense                                                                            Intense                                                                            Intense                     Fogging.sup.(14) (RH 50%)                                                                   ×                                                                             ×                                                                            ⊚                                                                   ⊚                                                                   ⊚                                                                   ⊚                                                                   ⊚                                                                   ⊚            Fogging (RH 80%)                                                                            ×                                                                             ×                                                                            ⊚                                                                   ⊚                                                                   ⊚                                                                   ⊚                                                                   ⊚                                                                   ⊚            Inside or outside the scope of                                                              Outside                                                                             Outside                                                                            Inside                                                                             Inside                                                                             Inside                                                                             Inside                                                                             Inside                                                                             Outside                     the present invention                                                         __________________________________________________________________________                  Example No.                                                                   9     10    11    12    13    14    15                          __________________________________________________________________________    Properties of toner particles                                                 Anti-blocking.sup.(8) (RH 50%)                                                              ×                                                                             ×                                                                             ⊚                                                                    Δ                                                                             Δ                                                                             ⊚                                                                    ⊚            Anti-blocking (RH 80%)                                                                      ×                                                                             ×                                                                             ⊚                                                                    ×                                                                             ×                                                                             ⊚                                                                    ⊚            Grindability.sup.(9)                                                                        A     A     D     A     A     D     D                           Fixing property                                                               Offset occurence                                                                            190   180   240<  240   230   240<  240<                        temperature.sup.(10) (°C.)                                             Lower temp. of fixing (°C.)                                                          140   140   200   160   180   190   200                         Temp. range of fixing (°C.)                                                           50    40    40<   80    50    50<   40<                        Resistance to dioctyl                                                                       ×                                                                             ×                                                                             ⊚                                                                    ⊚                                                                          ⊚                                                                    ⊚            phthalate.sup.(12)                                                            Image                                                                         Image intensity.sup.(13)                                                                    Intense                                                                             Intense                                                                             Intense                                                                             Intense                                                                             Intense                                                                             Intense                                                                             Intense                     Fogging.sup.(14) (RH 50%)                                                                   ×                                                                             ×                                                                             ⊚                                                                    ⊚                                                                          ⊚                                                                    ⊚            Fogging (RH 80%)                                                                            ×                                                                             ×                                                                             ⊚                                                                          Δ                                                                             ⊚                                                                    ⊚            Inside or outside the scope of                                                              Outside                                                                             Outside                                                                             Outside                                                                             Outside                                                                             Outside                                                                             Outside                                                                             Outside                     the present invention                                                         __________________________________________________________________________

What is claimed is:
 1. A resin for toner of electrophotography which comprises a urethane-modified polyester resin (D) having a glass transition temperature ranging from 40° to 75° C., which is obtained by reacting a resin mixture composed of a polyester resin (A) having a number-average molecular weight of 1,000 to 15,000 and a hydroxyl value of 10 to 100 and a polyester resin (B) having a number-average molecular weight ranging from 1,000 to 5,000 and a sum of acid value and hydroxyl value of less than 10, in a weight ratio of (A)/(B) ranging from 20:80 to 60:40, with 0.3 to 0.99 molar equivalent of an isocyanate compound (C) per one molar equivalent of hydroxyl group of the polyester resin (A).
 2. The resin of claim 1 wherein the polyester resin (A) is one obtained by polycondensation of a polycarboxylic acid and polyhydric alcohol.
 3. The resin of claim 2 wherein the polycarboxylic acid is an aliphatic dibasic acid, an aromatic dibasic acid or a lower alkyl ester thereof.
 4. The resin of claim 2 wherein the polyhydric alcohol is a bisphenol A-propylene oxide adduct.
 5. The resin of claim 1 wherein the number average molecular weight of the polyester resin (B) ranges from 2,000 to 4,000.
 6. The resin of claim 1 wherein the sum of the acid value and the hydroxyl value of the polyester resin (B) is less than
 8. 7. The resin of claim 1 wherein it is obtained by reacting the isocyanate compound (C) in an amount ranging from 0.5 to 0.95 molar equivalent per molar equivalent of hydroxyl group of the polyester resin (A).
 8. A toner composition for electrophotography comprising the resin of claim
 1. 9. The resin of claim 1 wherein the polyester resin (B) is obtained by polycondensation of a polycarboxylic acid and a polyhydric alcohol.
 10. The resin of claim 9 wherein the polycarboxylic acid is an aliphatic dibasic acid, an aromatic dibasic acid or a lower alkyl ester thereof.
 11. The resin of claim 9 wherein the polyhydric alcohol is a bisphenol A-propylene oxide adduct. 